Burner arrangements for firing fluidic fuels are used inter alia to operate gas turbines in power plants and other large machine applications. What are known as dual fuel burners are used in particular here, being provided optionally or combined to fire liquid and gaseous fuels, for example natural gas and fuel oil.
The burner arrangements have correspondingly large dimensions and feature a complex structure with a number of fuel supply ducts. Thus for example a centrally disposed smaller dimensioned pilot burner with its own fuel supply and air supply is frequently used to stabilize the flame of a large main burner, which is disposed around the pilot burner. The large main burner is mainly operated in lean mixture mode with excess oxygen to achieve more favorable emission values. However lean mixture mode means that the flame of the main burner is subject, at least in certain operating states, to fluctuations which are compensated for by a constantly igniting action of the pilot burner. Such a burner arrangement is set out for example in EP 0 580 683 B1.
One challenge with such burners is the mechanical stress resulting due to irregular thermal distribution in the walls of the metal housing, known as the hub, in which the annular supply ducts of the gas and oil energy carriers are disposed relatively close to one another. An annular gas chamber feeds the main burner on the input side in relation to the flow direction of the incoming air upstream of what are known as the swirl blades which swirl and mix the air flow with the combustion gas or through the swirl blades. An oil supply is also present, being generally disposed closer to the burner output than the gas supply. It comprises an annular oil chamber and an oil supply duct leading to the annular chamber, said duct being disposed in the hub wall between the annular gas chamber and the pilot burner.
Since gas is less dense than oil, it takes up a larger cross section, with the result that the dimensions of the gas supply are much larger than those of the oil supply. The part of the burner hub with the gas supply therefore has a larger outer surface facing the air duct than the oil supply. The air supply is effected with precompressed air, which has passed through a compressor, with the result that due to compression said supplied air has a temperature that is already above 400° C. The region of the burner hub with the gas supply is therefore quickly heated to a temperature in the region of above 400° C. and remains at this operating temperature. The oil supply duct leading to the annular oil chamber in contrast is further away from the hot air supply duct so that the oil in the oil supply duct is barely heated and therefore only has a temperature of around 50° C.
Since on the one hand the burner hub is significantly heated in the region of the annular gas chamber and on the other hand the adjacent oil supply duct is much cooler, the wall between the annular gas chamber and the oil supply duct is subject to a large temperature gradient. The temperature gradient causes thermal stress which shortens the service life of such burner hubs and makes it necessary to use a high-quality material with the costs this entails. Such stresses also occur in other regions where a cold fuel is carried through a hot hub region.